This invention relates generally to digital motor control systems and, more particularly, to a system for establishing the length of a driven interval of a motor driven mechanism. For example, one use for which the invention is particularly well suited is the control of the drive motor of a wavelength scanning mechanism in a radiant energy analyzer, such as a spectrophotometer, across a wavelength scan interval of a predetermined length.
In presently available radiant energy analyzers, the length of the wavelength scan interval traversed by a motor driven scanning mechanism is most commonly established in analog fashion by presetting first and second potentiometers on the instrument control panel to first and second respective limit voltage levels defining the upper and lower limits of the wavelength scan interval. This approach requires two voltage comparators to compare the two preset voltage levels with a scan position voltage tapped from a variable potentiometer coupled to the scanning mechanism, the scan position voltage level defining the actual position of the scanning mechanism in the scan interval. When the scan position voltage equals either of the two preset limit voltage levels, a scan limit has been reached and the appropriate comparator generates a motor control signal to inhibit, reverse, or otherwise control the wavelength scan motor.
In an analyzer of the above type, a motor driven chart recorder is typically employed to record an optical spectrum for the duration of the wavelength scan interval. The chart drive motor is synchronized with the wavelength drive motor to insure that the recorded spectrum corresponds to the wavelength spectrum actually scanned. It is desirable to increase the versatility in such analyzers to enable an operator to repetitively scan a selected wavelength interval and to record the output waveforms serially on successive portions of the chart recorder paper or, alternatively, to overlay the output waveforms on a single portion of the paper. In the latter case, by overlaying waveforms the change with time of a sample being analyzed can be readily shown, as is often desirable when the sample is undergoing a chemical reaction. In addition to the above, it is further desirable to enable the operator to manually initiate each successive scan or, alternatively, to provide for continuous, repetitive scanning and recording without the need for operator intervention.
Unfortunately, however, analog control of such analyzers is complex and the analog setting of the wavelength scan interval is time consuming and relatively inaccurate. Operator set-up of an analog system is a trial and error procedure which typically involves setting the voltage limits of the scan interval, performing successive trial scans, observing the actual interval scanned during each trial, and adjusting the voltage limits, as necessary, with each trial until the desired scan interval is obtained. As a result, an analog system lacks the accuracy and versatility needed for efficient and rapid set-up and execution of a scanning operation. Moreover, such a system precludes an operator from readily and expeditiously changing the scan limits to operate the system over one or more different scanning intervals.